The field of the present invention is the synthesis of sulfoxides. More particularly, the present invention pertains to the selective oxidation of sulfides with a perborate or a percarbonate.
Sulfoxides have the general structure SOR1R2. Sulfoxides can be produced in a variety of ways. By way of example, sulfoxides can be prepared by (a) oxidizing sulfides, (b) transferring sulphinyl groups, (c) rearranging other sulfoxides, (d) rearranging sulphenate esters, and (e) reacting SO with polyenes (See, e.g., Durst, T., in Comprehensive Organic Chemistry, Chapter 11.6).
The formation of sulfoxides from the oxidation of sulfides has been accomplished with a wide variety of oxidizing agents: PhlCl2, ozone, chromic acid, DABCO Br2, t-BuOOH, H2O, m-ClC6H4CO3H, PhlO, HNO3, NalO3, N2O4 and BuOCl (Durst, supra). A problem apparent to the use of all of these oxidizing agents is over oxidation of the sulfides to sulfones. In other words, the product of oxidation of a sulfide is a mixture of sulfide, sulfoxide and sulfone.
There continues to be a need in the art, therefore, for a means of selectively oxidizing sulfides to produce sulfoxides without the production of sulfones.
The present invention provides a process for the selective oxidation of a sulfide to a sulfoxide. The present process results in formation of the sulfoxide and avoids production of other reaction products such as sulfones. The process includes the step of oxidizing a sulfide in a reaction mixture containing a perborate or a percarbonate as an oxidizing agent to form a reaction product that contains the sulfoxide. The perborate or percarbonate oxidizing agent is preferably in the form of an alkali metal perborate or percarbonate salt. The alkali metal is preferably sodium.
Oxidation preferably occurs at a pH of from about 0.5 to about 5.0 and more preferably from about 0.5 to about 1.0. The pH is maintained by adding acid to a mixture of the sulfide and the oxidizing agent. A preferred acid is hydrochloric acid. Oxidation is preferably carried out at a temperature of from about 60xc2x0 C. to about 90xc2x0 C. and, more preferably at a temperature of from about 70xc2x0 C. to about 80xc2x0 C.
A preferred sulfide for use in the present process is 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole. In a preferred embodiment, the 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole is dissolved in an aprotic, aromatic solvent such as toluene.
The present invention provides a process for making a sulfoxide from a sulfide while avoiding over-oxidation of the sulfide to a sulfone. The controlled oxidation process includes the step of selectively oxidizing the sulfide in a reaction mixture containing perborate or percarbonate as the oxidizing agent.
The oxidizing agent for the selective oxidation is a perborate or a percarbonate salt. Exemplary and preferred such salts are alkali metal salts such as sodium, potassium or lithium salts. Sodium salts are preferred. Thus, preferred oxidizing agents are sodium perborate or sodium percarbonate, both of which can be obtained from commercial sources. The molar ratio of oxidizing agent to sulfide is from about 1:1 to about 3:1. Preferably, that molar ratio is about 2:1.
The oxidation of the sulfide occurs in an acidic environment. Thus, in addition to the sulfide and the oxidizing agent, the reaction mixture contains an acid. The amount of acid used in the reaction mixture is that amount sufficient to maintain the pH of the reaction mixture below a level of about 1.0 during oxidation. Preferably, the pH of the reaction mixture is maintained at a pH level of from about 0.5 to about 1.0. Any suitable acid that does not adversely affect the oxidation reaction can be used. Preferably, mineral acids such as hydrochloric acid, sulfuric acid or nitric acid are used. Hydrochloric acid is most preferred.
The acid can be added either before or after the addition of the oxidizing agent. In a preferred embodiment, the sulfide is mixed with acid (e.g., 10 weight percent HCl) before the addition of perborate or percarbonate. Additional acid may be needed to maintain the pH within the desired range during oxidation.
Oxidation typically occurs at a temperature of from about 60xc2x0 C. to about 85xc2x0 C. and, preferably at a temperature of from about 70xc2x0 C. to about 80xc2x0 C. unspent oxidizing agent can be recovered and optionally recycled into the reaction mixture. Still further, a process of this invention can include the step of recovering the formed sulfoxide.
Any sulfide can be used in the process of the present invention. A preferred sulfide is 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole. When this sulfide is used, the formed sulfoxide is 2-(methylsulfoxyl)-5-(trifluoromethyl)-1,3,4-thiadiazole.
In contrast to the oxidation of sulfides with traditional oxidizing agents (e.g., peroxide), the use of perborate or percarbonate avoids the formation of sulfones and results in the selective, high yield production of sulfoxides in a safe manner.
The following Examples illustrate preferred embodiments of the present invention and are not limiting of the specification and claims in any way.